The present disclosure relates to a coordinate measuring machine for measuring spatial coordinates of a workpiece. Moreover, the present disclosure relates to a method for producing a coordinate measuring machine. Furthermore, the present disclosure relates to a method for measuring optical properties of an optical filter that is used in the coordinate measuring machine according to the disclosure.
Coordinate measuring machines serve for checking workpieces, for example as part of quality assurance, or determining the geometry of a workpiece completely as part of what is known as “reverse engineering”. Moreover, diverse further application possibilities are conceivable, such as, for example, process-controlling applications, too, in which the measurement technique is applied directly for on-line monitoring and regulating of manufacturing and processing processes.
In coordinate measuring machines, different types of sensors may be used to capture the workpiece to be measured. By way of example, sensors that measure in tactile fashion are known in this respect, as are sold by the applicant under the name “VAST XT” or “VAST XXT”. Here, the surface of the workpiece to be measured is scanned with a stylus, the coordinates of said stylus in the measurement space being known at all times. Such a stylus may also be moved along the surface of a workpiece in a manner such that a plurality of measurement points can be captured at defined time intervals in such a measurement process as part of a so-called “scanning method”.
Moreover, it is known to use optical sensors that enable non-contact capture of the coordinates of a workpiece or measurement object. The present disclosure relates to such a coordinate measuring machine comprising an optical sensor.
In optical dimensional metrology, great outlays regularly arise if the form of workpieces is intended to be measured with accuracies in the range of single micrometres. This is generally attributable to the fact that comparatively complex and heavy sensors are guided by comparatively complex machines along preplanned trajectories. Subsequently or in parallel, the optically captured information is then related to the spatial information provided by the machine actuator system, such that the surface of the object to be measured can be reconstructed.
One example of an optical sensor that may be used in an optical coordinate measuring machine is the optical sensor sold by the applicant under the product designation “ViScan”. An optical sensor of this type can be used in various types of measurement setups or coordinate measuring machines. Examples of such coordinate measuring machines are the products “O-SELECT” and “O-INSPECT”, which are sold by the applicant.
A camera with high-resolution lens is usually used as optical sensor in such optical coordinate measuring machines. In optical metrology, to put it simply, the shadow casting of the measurement object is evaluated. To that end, on the imaging of the measurement object on the camera chip, the black-white transition is linked with the position of the measurement object. This link between image and object can be produced by calibration of the optical unit.
A basic prerequisite in the procedure mentioned above, however, is that the shadow casting, that is to say the bright and dark locations in the imaging that is imaged on the camera chip, also actually corresponds to the profile of the measurement object. For this reason, such optical systems that are intended to be used for metrological purposes have stringent requirements not only in respect of the imaging system but also in respect of the illumination system. Therefore, the illumination is ideally adapted to the imaging system in order to be able to achieve the best possible measurement results.
In order to be able to ensure the abovementioned stringent requirements in respect of the illumination system, a telecentric illumination optical unit is often used in optical coordinate measuring machines. For space and/or cost reasons, however, said telecentric illumination optical unit may also be replaced by a surface luminous element of flat design. However, this measure then limits the measurement accuracy on account of reflections of the diffuse light at the measurement object primarily in the case of volume parts. In order, even in the case of such a construction of the coordinate measuring machine, to attain once again the range of measurement accuracy such as is achievable using a telecentric illumination optical unit, the surface luminous source of flat design may also be replaced or extended by other components.
EP 1 618 349 B1 describes for example a coordinate measuring machine comprising a transmitted-light illumination arrangement, wherein the transmitted-light illumination arrangement comprises an illumination body in the form of a surface luminous source embodied such that it radiates diffusely. In addition to the image processing sensor system and said transmitted-light illumination arrangement, the coordinate measuring machine comprises a filter arranged between the surface luminous source and the measurement object. Said filter has channel-like passage openings that are aligned parallel to the optical axis of the lens of the image processing sensor system and transmit only rays at less than a predefined limiting angle α with respect to the optical axis. The limiting angle α at which rays can pass through the passage openings has a value of, in principle, less than 10°, preferably less than 3°, possibly even less than 1°. In accordance with the teaching of EP 1 618 349 B1, said optical filter is intended to avoid extraneous light that might otherwise pass into the optical unit, that is to say the image processing sensor system. The intention is thereby to avoid imaging aberrations and thus also measurement errors, in particular when measuring rotationally symmetrical parts.
It has been found, however, that with the use of an optical filter as described in EP 1 618 349 B1, very narrow tolerances would have to be complied with in order to align the diffusely radiating light of the surface luminous source such that the measurement errors described above cannot arise. For small areas to be illuminated, process-reliable production and mounting of such an optical filter is possible potentially in a relatively simple manner. However, if the area to be illuminated is a relatively large area, for example an area in the range of 100×100 mm2, it is virtually impossible in practice to ensure the avoidance of measurement errors using the solution known from EP 1 618 349 B1. This is owing to the fact, in particular, that the optical filters used can scarcely fulfil the narrow tolerances required, owing to dictates of manufacturing.
The opening of the light cone that leaves one of the plurality of channel-like openings of the optical filter should typically have a value of less than 5°. The direction of the centroid ray, that is to say the direction of the light cone centre axis or light cone principal axis, should run perpendicularly to the mechanical surface of the filter. It goes without saying that these requirements must be met not just for one of the plurality of channel-like passage openings of the optical filter, but for all of the channel-like openings, that is to say must be identical over the entire surface of the optical filter.
Current measurements have revealed, however, that the above-described desired emission characteristic of the optical filter can scarcely be ensured, or can be ensured only with extremely high outlay, in practice for production engineering reasons. The measurements carried out by the applicant have revealed, for example, that although the emission characteristic of such an optical filter usually complies with the demanded 5° aperture angle of the light cones over the entire image field, the direction of the centroid rays, that is to say the centre axes of the light cones, is on no account aligned perpendicularly to the mechanical surface of the optical filter over the entire field of view. Instead, it has been found that this requirement (direction of the centroid rays perpendicular to the mechanical surface) not only is not met on average, but in addition is also different in a position-dependent manner.
In actual fact, therefore, with the use of an optical filter as proposed in EP 1 618 349 B1, the maximum possible quantity of light from the light source is not transmitted by the filter and picked up by the imaging optical unit or the optical sensor. In addition, on account of the above-described emission characteristic of the filter, undesired imaged patterns occur which adversely influence the measurement operation and subjective impression of the overall system.